Field of the Invention
The present invention relates generally to an automatic control scheme. More particularly the present invention relates to a method and apparatus for protecting a turbocompressor from surge by monitoring crucial vibration information and acting thereon.
Background Art
Compressor surge and stall represent unstable operating regimes of axial and centrifugal turbocompressors. Either mode of instability may lead to compressor damage. First, rotor vibration due to the unsteady flow of stall can cause flutter and associated fatigue. The flow reversal of surge results in an increase in temperature within the compressor. At the same time, the reversed flow and pressure variations between the intake and discharge ends of the compressor cause rapid changes in axial thrust, thereby risking damage to the thrust bearing and causing blades or vanes to rub on the compressor housing. Furthermore, abrupt speed changes may occur, possibly resulting in overspeed or underspeed of the compressor rotor.
The aerodynamics of rotating stall and surge have been investigated extensively. This research has led to common industry definitions of local stall, stage stall, stall zone, surge and rotating stall. Local stall is a flow separation or reversal in either an impeller or diffuser in a limited angular range. Stage stall is when a local stall increases to the point where one in a series of centrifugal impellers (and associated inlet and discharge stationary components) experiences reversed flow in part of its cross-sectional flow area. In any stalled condition, the overall flow is still in a forward, pressurizing direction. A stall zone is any cross-sectional area of an impeller or diffuser undergoing a flow perturbation and which manifests symptoms of a stall in a compressor. Surge is defined as the periodic oscillations of the bulk compressor flow and its associated periodic pressure swings. If these oscillations include flow reversals, deep surge is said to have occurred. Rotating stall comprises stall zones covering several blades and passages. The stall zones propagate circumferentially at a fraction of rotor speed. The number of stall zones and the propagating rates vary between compressors.
A compressor and traditional antisurge control system are shown in FIG. 1. Turbocompressor antisurge control methods of the prior art make use of thermodynamic information taken at the inlet and outlet of the compressor 100 driven by a driver 105. This information typically comprises at least a pressure differential signal gleaned from an obstruction flow meter 110 and transmitted by a flow transmitter 115, suction pressure, transmitted by a suction pressure transmitter 120, and discharge pressure, transmitted by a discharge pressure transmitter 130. These signals are fed into an antisurge controller 140 where the signals are analyzed and a closed loop response is calculated. This closed loop response determines the set point of an antisurge valve 150. Signals representing other thermodynamic data, such as one or more temperatures, may also be used by the antisurge controller. Mechanical parameters such as compressor rotational speed, inlet guide vane position, or discharge guide vane position may also be measured and transmitted to the antisurge control system. Typical compressor drivers 105 comprise steam or gas turbines and electric motors.
The antisurge valve 150 may be a recycle valve such as that shown in FIG. 1, or a blowoff valve 250, as illustrated in FIG. 2 and as might be used for air, nitrogen, and sometimes CO2 compressors. Surge is avoided or recovered from by increasing the flow rate through the compressor via opening the antisurge valve.
Traditionally independent of the antisurge control system, vibration data are taken at radial bearing locations, at the thrust bearing, and at other locations on the compressor to monitor movement and vibration of the compressor rotor or impeller shaft. During operation, the compressor shaft is held against a thrust bearing with slight movement depending on operating rotational speed and other conditions.
Compressor surge is described in several textbooks and many articles. One such textbook is Aircraft Propulsion by Saeed Farokhi (ISBN 978-0-470-03906), published by John Wiley and Sons, is hereby incorporated in its entirety by reference. Simply speaking, surge can be defined as a point where the compressor can no longer maintain an adequate pressure difference to continue forward flow, and a bulk flow reversal occurs. Detecting the rapid reversal in flow when using an obstruction flow meter is fairly straightforward. Some compressors, however, are not fitted with a flow meter of any kind. Further, sensors and transmitters can fail so other methods to detect surge and to provide antisurge control would be of value.
There is, therefore, a need for a method and apparatus to detect surge and protect a compressor therefrom using signals other than those of thermodynamic nature.